Advances in flat panel displays (FPD) technology have made high quality, large area, full color high resolution displays possible. Currently, liquid crystal displays (LCD) are the display of choice. A major drawback to LCD panels is their poor performance in low ambient light conditions. For example, reflective LCDs can only be used in high ambient light conditions because they derive their light from the ambient light, i.e. the ambient light is reflected by the LCDs. Some transflective LCDs are designed to operate in a transmissive mode and incorporate a backlighting arrangement for use when ambient light is insufficient. In addition, transflective displays have a certain visual aspect and some users prefer a bright emissive display. However, these types of displays are generally too large for practical use in very small devices, such as portable electronic devices and consume considerable power which adversely affects portable display applications.
Organic electroluminescent device (OED) arrays are emerging as a potentially viable design choice for use in small products, especially small portable electronic devices, such as pagers, cellular and portable telephones, two-way radios, data banks, etc. OED arrays are capable of generating sufficient light for use in displays under a variety of ambient light conditions (from little or no ambient light to bright ambient light). Further, OEDs can be fabricated relatively cheaply and in a variety of sizes from very small (less than a tenth millimeter in diameter) to relatively large (greater than an inch) so that OED arrays can be fabricated in a variety of sizes. Also, OEDs have the added advantage that their emissive operation provides a very wide viewing angle.
A drawback with OED devices are that they are difficult to drive using simple two-terminal schemes because of their lack of memory. The rise and decay time of an OED is very fast and does not have intrinsic memory. To overcome this problem, four terminal thin-film-transistor (TFT) devices have been developed to drive OED devices. These devices include two TFTs, a storage capacitor and an OED pad arranged on a substrate. The storage capacitor enables the excitation power to an addressed EL element to stay on once it is selected.
While successfully overcoming the above mentioned problem, the storage capacitor process and deposition are very complicated and difficult to achieve in a fabrication process. In these devices, a capacitor is formed by the gate electrode, acting as the bottom electrode of the capacitor, separated from an overlying top electrode by a gate oxide insulating layer. The top electrode is connected to the source region. An example of this structure can be seen in European Patent Office Publication No. EP O 717,445 A2, published Jun. 19 1996. Several problems are apparent in this type of device, namely, the process is complicated and leakage is possible due to the sharp edge at the anode of the OED formed by the capacitor process.
In spite of all the advantages of OEDs there are still some draw backs to their use. One of the drawbacks is the fact that light emitted by OEDs is dependent upon the amount of current applied to the OED. Thus, to produce sufficient light for use in displays substantial amounts of current must be applied. As a typical example, with a 64.times.32 array of devices forming a four line by 11 character display the following devices will use approximately the listed amount of power to produce equal light out. If the devices are semiconductor light emitting diodes the display will require approximately 1 Watt (w) of power; organic light emitting diodes will require approximately 150 mw of power; LCDs with a backlight will require approximately 120 mw of power; and reflective LCDs will require approximately 20 mw of power.
Conventional TFT-OED displays suffer from several limitations, including insufficient brightness, relatively high power consumption, and poor chromaticity. In particular, the red-color OED usually shows poor efficiency and short lifetime, which do not meet with the requirements of a full color display. An additional problem in the use of OEDs in displays is the generation of the colors necessary to achieve a full color display. Red, green and blue OEDs can be fabricated but they require different organic materials and, thus, each color must be fabricated separately. Furthermore, the colors achieved are not a pure primary color, but have a relatively broad spectrum.
Accordingly, it is highly desirable to provide a new and improved light emitting apparatus and method of fabrication.
It is another purpose of the present invention to provide a new and improved light emitting apparatus with enhanced performance.
It is a further object of the present invention to provide a new and improved method of fabricating a light emitting apparatus with a simplified process.
It is a further object of the present invention to provide a new and improved method of fabricating a light emitting apparatus having a microcavity fabricated with a simplified process.
It is yet another object of the present invention to provide a light emitting apparatus with a capacitor.
It is yet a further object of the present invention to provide a light emitting apparatus with a microcavity for light enhancement with reduced power consumption.